Can something be evident and incredible at the same time? Certainly, if you are in denial. Until Russian troops landed in the Crimea many Russians were in denial about Vladimir Putin. They believed he was all bark and no bite.

Not that Putin had kept his intentions secret. He has always denied the idea that the Soviet Union was a colonising power; furthermore, he called the breakup of the USSR "the greatest geopolitical catastrophe of our time".

He has annexed chunks of Georgia, most recently by means of a military invasion in 2008. But there are two differences between now and the war in Georgia. Technically, it was not Putin but Dmitry Medvedev who was nominally president when Russia invaded Georgia. More importantly, Russian liberals were not rooting for their fellows in Georgia during that war; indeed, they were scarcely aware of the political struggles within the country.

Ukraine is different: for three months, Russians had been watching the stand-off, and the oppositionally minded were strongly identifying with the anti-Yanukovych forces in Kiev.

Perhaps the last time the Russian intelligentsia watched the internal struggle in another country this intently was in 1968 during the Prague Spring, when they hoped the Czechs would succeed in building what they called "socialism with a human face". They also believed it would hold out the promise of something better for life in the Soviet Union. In August 1968, the Soviets invaded Czechoslovakia, quashing the Prague Spring. In Moscow, seven people came out to protest against the invasion; they were arrested and the modern dissident movement was born.

The parallels end there. It's unlikely that what's happening in Ukraine will foment a new protest movement in Russia: the ongoing crackdown on civil society makes the cost of protest too high. Still, the Crimean invasion is a landmark in Russian domestic politics.

It signals a loss of innocence: no longer will Russians be able to think that Putin merely feels nostalgic for the USSR. It also signals ever greater polarisation of Russian society: in addition to all the other lines along which Russians are divided and across which civilised dialogue is impossible, there is now the chasm between supporters and opponents of the planned annexation. It also means the political crackdown in Russia will intensify further.

These clear and tragic consequences obscure the challenge the new Crimean war poses to Russia's post-imperial consciousness. "I can be reasonable about everything, but I cannot give up the Crimea," was a line from the late Galina Starovoitova, who as Boris Yeltsin's adviser on nationalities policy, oversaw Russia's first attempts at releasing its colonies.

She meant that, like just about every Russian, she felt the Black Sea resort area was part of her birthright, whatever the maps may say. Most, if not all, Russians harbour this Crimean exceptionalism, even if they belong to the minority that otherwise rejects Soviet nostalgia.

If Russia functioned as a society with rule of law and some common understanding of its complicated history, the inhibition against acting on this exceptionalist impulse would come from the top. But with the government sending troops into the Crimea, it is up to individual Russians to find the arguments and, even more difficult, the motivation to resist the aggression.

Masha Gessen is the author of The Man Without A Face: The Unlikely Rise of Vladimir Putin